1. Field of the Invention
The present invention relates to a radar or, more particularly, to a radar suitable for a vehicle and usable to help prevent collision or to realize automatic cruise control or automatic operation.
2. Description of the Related Art
Various forms of radar, including a frequency modulated-continuous wave (FM-CW) form and a pulsed-Doppler form, are adaptable to a radar that measures a distance between a vehicle and an object or a relative speed of an object to a vehicle. Above all, the FM-CW radar has the advantages of being relatively compact and inexpensive in circuitry and of simultaneously measuring both a distance between moving entities and a relative speed. Therefore, the FM-CW radar has been adopted for vehicles.
In general, the radar employs, as described in, for example, Japanese Unexamined Patent Publication No. 2001-183450, an analog-to-digital (A/D) converter because an analog signal received by a receiving antenna is converted into a digital signal and then processed digitally.
Incidentally, the A/D converter falls into a single-polarity power type that receives a voltage ranging from 0 V to any positive value (for example, ranging from 0 V to 5 V) and a dual-polarity power type that receives a voltage ranging from any negative value to any positive value. Normally, A/D converters incorporated in microprocessors and DSPs are of the single-polarity power type.
As for the radar, when a signal must be analyzed using a single-polarity power type A/D converter, a signal component must be appropriately biased (with a dc voltage). In this case, if fast Fourier transform (FFT) is adopted for the signal analysis, a low-frequency signal component may not be detected because of the adverse effect of a dc voltage, or information may be distorted.
Moreover, if a dual-polarity power type A/D converter is employed, a filter or the like is used to remove a dc component of a signal, and the resultant signal is analyzed through Fourier transform or the like. In this case, when a window function or the like is applied to the signal, a dc component may be produced. This adversely affects a low-frequency signal component.